The present invention relates to cold-cathode field emission displays, and, more particularly, to baseplates for field emission displays that have internal current-limiting devices.
Field emission displays (FEDs) are packaged vacuum microelectronic devices that are used in connection with computers, television sets, camcorder viewfinders and other electronic devices requiring flat panel displays. FEDs have a baseplate and a faceplate juxtaposed to one another across a narrow vacuum gap. In large FEDs, a number of spacers are positioned between the baseplate and the faceplate to prevent atmospheric pressure from collapsing the plates together. The baseplate typically has a base substrate upon which an array of sharp, cone-shaped emitters are formed. The emitters in each of the rows or columns of the array may be connected to each other and isolated from the emitters in the other rows or columns, respectively. An insulator layer is positioned on the substrate having apertures through which the emitters extend, and an extraction grid formed on the insulator layer around the apertures. The faceplate has a substantially transparent substrate, a transparent conductive layer disposed on the transparent substrate, and a cathodoluminescent material deposited on the transparent conductive layer.
In operation, a potential is established across the extraction grid and the emitters to extricate electrons from the emitters. The electrons pass through the holes in the insulator layer and the extraction grid, and impinge upon the cathodoluminescent material in the desired pattern. In the event that the emitters in a row or column are interconnected and isolated from the emitters in other rows, the emission of electrons from the emitters in individual rows or columns can be controlled.
FEDs may also have a current control device to switch or limit the amount of current that can flow through the emitters. Limiting the emitter current is important because an excessively high current generates a significant amount of heat in the emitters, which may damage or destroy the emitters. Conventional current-limiting devices may be fabricated on the base substrate or a separate substrate formed within the vacuum chamber of an FED. Switching the emitter current is performed in various emitter addressing schemes with several row lines or column lines coupled to switches formed on the baseplate by implanting various materials into the base substrate. The base substrate in such FEDs is often a complex, expensive component to manufacture, and forming current control devices on the base substrate makes the base substrate even more complex. Accordingly, forming current control devices on the base substrate is often more time-consuming and costly than forming the same devices on a separate substrate. Thus, it is often more desirable to fabricate current control devices on a separate substrate
Conventional processes for making an FED having a separate substrate for a current control device typically involve forming the emitters on top of a conductive material, and then masking the emitters with a protective layer. The separate substrate for the current control devices is then deposited on the unprotected areas, after which the mask is removed from the emitters. Subsequently, an insulator layer is disposed over the emitters and the separate substrate, and a layer of conductive material is disposed over the insulator layer. The area over the separate substrate is then masked, and the extraction grid is formed from the conductive layer of material by a chemical mechanical planarization (CMP) process. Conventional baseplates are designed with the understanding that it is desirable to form the grid from a highly conductive material such as a metal or conductive polycrystalline. Finally, the mask over the separate substrate is removed. The separate substrate may be made from a material having sufficient resistivity to act as a resistor, thus allowing the separate substrate itself to act as a current-limiting device. A power source is connected to the separate substrate such that electrons flow to the emitters through the separate substrate.
One problem with forming a separate substrate for the current control device is that it increases the cost of producing an FED. In addition to the basic steps of forming an FED, the production of a separate substrate for a current control device requires several masking steps and the step of depositing the separate substrate itself. Moreover, because the separate substrate for a current control device is often made from a different material than the other components of an FED, it requires separate and generally less efficient handling procedures. Therefore, it would be desirable to develop a process for manufacturing a baseplate with a separate substrate for a current control device that uses fewer steps and fewer materials.
The baseplate of the present invention has a supporting substrate with a primary surface upon which an array of emitters is formed. An insulator layer with a plurality of openings aligned with respective emitters is disposed on the primary surface, and an extraction grid with a plurality of cavity openings aligned with respective emitters is deposited on the insulator layer. The extraction grid is made from a silicon based layer of material. A substrate that is separate from the supporting substrate is formed from the same silicon based material used for the extraction grid and is electrically isolated from the extraction grid. A current control device on the separate substrate is electrically connected between the emitters and a voltage source such that electrons from the voltage source flow through the current control substrate to the emitters. In one embodiment, the silicon based material of the grid and current control substrate is sufficiently resistant to allow the current control substrate itself to limit the current to the emitters.
The inventive method for manufacturing a baseplate of the present invention includes forming emitters on a supporting substrate, disposing a dielectric material over the emitters and the supporting substrate, and depositing a silicon based material on the dielectric material. The silicon based material is deposited such that it has a first section positioned over at least a portion of the emitters and a second section that is contiguous with the first section. A number of cavity openings are then fabricated in the first section such that each cavity opening is aligned with a corresponding emitter. The layer of silicon based material is then processed to electrically isolate the first and second sections from one another. The dielectric material in the cavity openings of the grid and adjacent to the emitters is removed to open the emitters to the holes. The second section of the silicon based material provides a separate substrate on which a current control device may be formed to control the current flowing to the emitters.